where can I see the multi-level BOM weight report?
There isn't a multi-level BOM weight report in an OOTB database. However, you can look at the existing BOM Costing Report as an example of how you could build your own weight report. You could even run a Save As on the BOM Costing Report to create a copy and then change all references to the cost property to weight and all references to the cost_basis property to weight_basis.
Aras Labs Software Engineer